Well, if you also want to sell Final Fantasy XV-2 and Final Fantasy XV-3 and make as much money as possible then, yeah, you do the work needed to get the game working on 2 consoles that, unlike the ps3 and 360, are at pretty comparable levels of power and maximize your profits. It isn't as if your xb1 sales are cannibalizing whatever your ps4 sales would be and any real hard work needed to get the game up and running on the ps4 will transfer over to the xb1 one.
Hell, just drop the resolution on the XB1 version if that's so hard to pull off and call it a night. It would still be a better effort than what they put into FFXIII on the 360 and that sold 500k on release and was a total piece of shit port. Again, it's easy money and a bad move not to have it come out on the xb1, there's no real justifying that not releasing it on that platform is a good move outside of possibly some weird console war hangups that some people may have.
It'll be hard to weigh the pros and cons without actual information. Maybe we should start by quantifying how well the Xbox version would have to do in order to justify it.
I think these games will come out on both consoles for the reasons you mentioned, but this gen there is much less incentive to do it. First off the XBO doesn't have a commanding lead in NA or the UK like the 360 did. With the PS4 ahead in those markets I'm skeptical XV on XBO will come anywhere near replicating XIII's sales on 360.
Also while the two consoles are more architecturally similar this gen, they're further apart in performance. The 360 versions of FFXIII-2 and Lightning Returns were practically at parity with the PS3 versions, whereas the PS4 will probably always have a glaring lead in performance and visual quality.
There might also be additional resource costs associated with supporting the Xbox that we're just not able to factor in. Perhaps certain effects or engine features are scaled back or removed from the game entirely. PS4 is more adept at leveraging asynchronous compute, but it's questionable whether SE will take advantage of this when they have to support a platform that doesn't have the same capability. There's probably some impact on development time trying to ship two versions. Maybe the extra time would be better spent working on airship traversal or adding character switching.
If cutting one version could result in a better game for *95% of the potential audience I'd say the Xbox version wasn't worth it in the end.
*hypothetical estimate, no actual basis in reality